1. Field of the Invention
The present invention relates to a nonreflective waveguide terminator and a waveguide circuit which transmit microwave and millimeter-wave signals.
2. Description of the Related Art
FIG. 16 is a perspective view showing the structure of a nonreflective waveguide terminator disclosed in, for example, Japanese Unexamined Patent Application Publication No. 5-243817. In a rectangular waveguide 1 having one short-circuited end, electromagnetic-wave absorbing plates 3 for absorbing a high frequency magnetic field are disposed on wall surfaces 2 which are parallel to an electric field inside the rectangular waveguide 1.
FIG. 17 is a perspective view showing the structure of a nonreflective waveguide terminator disclosed in Japanese Unexamined Patent Application Publication No. 5-243817. In a rectangular waveguide 1 having one short-circuited end, a tapered electromagnetic-wave absorber is disposed in the center of the rectangular waveguide 1 in a propagation direction of radio waves.
Next, the operation of the nonreflective waveguide terminator shown in FIG. 16 is described below.
Assuming that the left end of the waveguide 1 in FIG. 16 is an input port, an incident microwave signal is gradually absorbed by the electromagnetic-wave absorbing plates 3 disposed on planes parallel to the electric field in the waveguide 1. The electric field distribution is concentrated in the center of the cross-section of the rectangular waveguide 1. Thus, by reducing the thickness of the electromagnetic-wave absorbing plates 3, reflection of the microwave signal can be suppressed to a low level. Accordingly, in the design of the electromagnetic wave absorbing plates 3, the plate thickness is reduced and the length of the electromagnetic-wave absorbing plates 3 in the propagation direction of the microwave signal is set in order to obtain a predetermined absorption for no reflection according to the reduced thickness. This structure is suitable for a nonreflective waveguide terminator which is rectangular and for high power because it has a reduced electromagnetic wave absorption per unit volume and an enlarged radiation area in contact with the wall surfaces 2, which are parallel with the electric field.
Next, the operation of the nonreflective waveguide terminator shown in FIG. 17 is described below.
Assuming that the left end of the waveguide 1 in FIG. 17 is an input port, an incident microwave signal is gradually absorbed by the tapered electromagnetic-wave absorber 4 while reducing reflection. Compared with the nonreflective waveguide terminator in FIG. 16, the nonreflective waveguide terminator in FIG. 17 has a low radiating effect since it has a smaller area in contact with the internal wall surfaces. However, since the tapered shape determines the reflection characteristics, there is a possibility that the length of the nonreflective waveguide terminator in the propagation direction of the microwave signal may be reduced.
In the structure of the nonreflective waveguide terminator in FIG. 16, in the case of reducing reflection in the electromagnetic-wave absorbing plates 3, the plate thickness must be reduced for the purpose of lowering discontinuities caused by the end faces of the electromagnetic-wave absorbing plates 3. In the case of obtaining predetermined reflection characteristics, the length of the electromagnetic wave absorbing plates 3 in the propagation direction of the microwave signal must be increased so that the required electromagnetic wave absorption is obtained. Accordingly, the nonreflective waveguide terminator in FIG. 16 has a problem in that the length of the electromagnetic-wave absorbing plates 3 must be increased in order to obtain a predetermined reflection level or less, even if it is more than enough for the length required for high-power tolerant performance such as heat radiation performance. Also, the nonreflective waveguide terminator in FIG. 17 suffers from a problem of high production costs because it is difficult to perform processing of the tapered electromagnetic wave absorber 4.